Multi-File Builds and Headers

Larger programs usually place public interfaces in a header, implementations in a source file, and calls in a main file. Each source file is compiled independently into an object file, then the linker combines those objects into an executable.

math_utils.h — declares the shared interface

math_utils.c — provides function definitions

main.c — calls the public functions

Headers and include guards

A header guard uses a unique uppercase name. The first inclusion defines that name; later inclusions see a false ifndef condition and skip the body. Headers should contain declarations, not ordinary function definitions that would create multiple copies.

math_utils.h

#ifndef MATH_UTILS_H
#define MATH_UTILS_H

double circle_area(double radius);
int clamp(int value, int minimum, int maximum);

#endif

Implementation and main files

math_utils.c

#include "math_utils.h"

#define PI 3.1415926

double circle_area(double radius) {
    return PI * radius * radius;
}

int clamp(int value, int minimum, int maximum) {
    if (value < minimum) {
        return minimum;
    }

    if (value > maximum) {
        return maximum;
    }

    return value;
}

main.c

#include <stdio.h>
#include "math_utils.h"

int main(void) {
    double radius = 3.0;

    printf("%.2f\n", circle_area(radius));
    printf("%d\n", clamp(125, 0, 100));

    return 0;
}

Compile separately, then link

The -c option compiles without linking and produces .o object files. The final command links those objects into calculator. After changing only main, you can recompile main and link again.

gcc -Wall -c math_utils.c -o math_utils.o
gcc -Wall -c main.c -o main.o
gcc main.o math_utils.o -o calculator

# Or compile and link in one command:
gcc -Wall main.c math_utils.c -o calculator